This invention generally relates to closures for openings and more particularly relates to a shielded access door for a nuclear steam generator manway port having a beam of radiation streaming therethrough.
Although devices for covering nuclear steam generator manways during maintenance operations are known, it has been observed by applicant that these devices have a number of operational problems associated with them. However, before these problems can be appreciated, some background is necessary as to the structure and operation of a typical nuclear steam generator.
In this regard, a typical nuclear heat exchanger or steam generator generates steam when heat is transferred from a heated and radioactive primary fluid (i.e., water) to a non-radioactive secondary fluid (i.e., also water) of lower temperature. The primary fluid flows through a plurality of inverted U-shaped tubes, which are received through holes in a plurality of spaced-apart support plates while the ends of each tube are received in a tubesheet disposed below the bottom-most support plate. The interior of the steam generator below the tubesheet (i.e., the channel head) in combination with the interior of the tubes define a radioactive primary side of the steam generator while the interior of the steam generator above the tubesheet defines a nonradioactive secondary side of the steam generator.
During operation of the steam generator, the secondary fluid flows across the exterior surfaces of the tubes as the primary fluid flows through the tubes. The walls of the tubes function as heat conductors for transferring heat from the heated primary fluid flowing through the tubes to the secondary fluid of lower temperature flowing across the exterior surfaces of the tubes. Thus, the tubesheet and the U-shaped tubes hydraulically isolate the primary side from the secondary side of the steam generator while thermally connecting them together. The steam generator also includes a plurality of circular manway openings in communication with the channel head for allowing access to the interior of the steam generator in order to perform maintenance on the steam generator, which manways are sealingly covered by manway hatch covers during operation of the steam generator. Surrounding each manway is a manway flange to which the hatch cover is sealingly bolted.
For safety reasons, the tubes are designed to be leak-tight so that the radioactive primary fluid remains separated from the nonradioactive secondary fluid to avoid commingling the primary fluid with the secondary fluid. Occasionally, however, the steam generator tubes may degrade due to stress and corrosion occurring during operation and thus may not remain leak-tight. This is undesirable for safety and economic reasons.
Therefore, during maintenance of the steam generator, the steam generator is taken off-line and the tubes are inspected for degradation using various inspection devices, such as eddy current or ultrasonic probes inserted into the tubes. These probes travel along the inside surface of the tube for inspecting the tube. However, if the inner surface of the tube is not sufficiently dry during the inspection process, the probe may slip on the wet inner surface of the tube due to lack of traction, which slippage can lead to inaccurate instrument readings. Therefore, it is important that the inner surface of the tube is sufficiently dry to satisfactorily perform the inspection process. Moreover, the maintenance process may also include removing radioactive sludge deposits and other particulate matter from the interior surfaces of the steam generator because such sludge and particulate matter may damage the tubes during operation. However, the sludge removal processes may increase the amount of any air-borne radioactive micro-particles that may be already suspended in the atmosphere inside the steam generator. These air-borne radioactive particles are preferably exhausted from the steam generator before maintenance personnel service the steam generator in order to reduce radiation exposure to the maintenance personnel. Exhausting the air-borne radioactive particles from the steam generator is important because such particles can adhere to the clothing of maintenance personnel, thereby resulting in a need to expend more effort and expense to decontaminate the clothing worn by the maintenance personnel.
Of course, before the maintenance process is begun, the primary and secondary fluids are drained and the manway hatch covers covering the steam generator manways are removed for allowing access to the steam generator interior. However, because the interior of the steam generator is radioactive, a beam of radiation "shines" or streams through the manway when the manway hatch cover is removed. This beam of radiation may have an intensity of approximately seven thousand mR (i.e., milliroentgen) at the manway. Exposing maintenance personnel to such a radiation level for prolonged time periods is undesirable for health physics reasons. Therefore, shielding covers have been used in the art to suitably cover the manway for reducing the radiation emissions in order to protect maintenance personnel from the radiation as they service the steam generator.
However, manway shielding covers are relatively heavy because the shielding material comprising the cover is usually made of a metal of relatively high mass number, such as lead or the like, in order to provide sufficient shielding. Such covers may attain a weight of approximately 60 pounds or more. The heavy weight of the covers inhibits the ability of maintenance personnel to readily lift and position the covers over the manway. Therefore, a problem in the art is to provide a manway cover that is light weight so that it can be readily lifted and positioned to cover the manway.
Moreover, a well known characteristic of lead is that it is relatively soft or malleable. Therefore, the surface of the lead covers are easily dented in use and hence are not usually smooth. The rough or dented surface of such lead covers provides sites for accumulation of the air-borne radioactive particulate matter that may be suspended in the atmosphere of the interior of the steam generator. This accumulation of radioactive particulate matter on the surface of the lead covers requires greater effort and expense to decontaminate the covers following steam generator maintenance. Therefore, another problem in the art to provide a manway cover that can be readily decontaminated.
In addition, insulation may surround the exterior surface of the steam generator for inhibiting heat loss from the exterior surface of the steam generator. This heat loss is undesirable because it results in a reduction in the thermal efficiency of the steam generator. Such insulation typically results in approximately a six to eight inch interference extending outwardly from the face of the manway flange. This interference requires a portion of the insulation surrounding the manway flange to be removed in order to provide space to position the shielding cover over the manway. Therefore, yet another problem in the art is to provide a shielding cover that does not require a portion of the insulation surrounding the manway flange to be removed in order to position the shielding cover over the manway.
Shielding and exhaust covers for nuclear steam generator manways are known. A shielding cover and an exhaust cover which are mountable on the primary manways of a nuclear steam generator for protecting maintenance personnel from potentially harmful radiation during maintenance operations is disclosed in U.S. Pat. No. 4,948,981 titled "Primary Manway Shielding And Exhaust Covers For A Steam Generator" issued Aug. 14, 1990 in the name of Wayne R. Wallace, et al. and assigned to the assignee of the present invention. This patent discloses shielding and exhaust covers including interchangeable mounting flanges having brackets for mounting a dual rail track assembly. The shielding and exhaust covers are each adapted to readily receive the dual rail track so as to be slidably movable from an open position to a closed position, and to allow for ease in the initial positioning of the covers. The exhaust cover itself includes a central opening with a nozzle extending outwardly from the cover. A shielding plate is positioned on the inner side of the exhaust cover so as to allow air to pass therebetween and out of the nozzle. The mounting flange allows the shielding and exhaust covers to be interchanged on the same manways. Although this patent discloses a shielding and an exhaust cover mountable on the primary manways of a nuclear steam generator, this patent does not appear to disclose a shielding cover that allows access to the manway as it simultaneously shields against the radiation and therefore does not appear to disclose a suitable shielded access door for a port having a beam of radiation streaming therethrough, as described and claimed hereinbelow.
Therefore, what is needed is a shielded access door for a nuclear steam generator manway port having a beam of radiation streaming therethrough.